The possible role of viruses and interferon (IFN) in human immunopathologic process has been studied. We have extended our studies on the immunoregulatory actions of IFN by studying patients with lymphoproliferative disorders. We have observed a patient with a lymphoproliferative disease whose lymphocytes spontanteously produce IFN-gamma in vitro, a phenomenon which has not previously been recognized. The patient has a proliferation of a mature T cell identified as a Tg cell with cytotoxic activity. The precise role of IFN in the pathogenesis and/or expression of this hemtologic disorder is uncertain at present.